Optical printer

ABSTRACT

In an optical printer apparatus designed so that an optical head ( 100 ) having an LED light source ( 110 ) therein is moved relatively to a sensitized sheet ( 500 ) and an image is formed by emitting a plurality of color light beams in regular order from the LED so that the light beams are focused at given spaces on the sensitized sheet, an image pitch P for the color light beams is substantially equal to an integer multiple of the maximum exposure distance D.

TECHNICAL FIELD

The present invention relates to an optical printer apparatus capable ofrelatively moving on a sensitized sheet to expose it with given timing,thereby forming an image, and more specifically, to a technique forcontrolling the exposure timing of the optical printer apparatus.

BACKGROUND ART

Disclosed in Japanese Patent Application Laid-open No. 2-169270 is anoptical printer apparatus in which an optical head is relatively movedon a sensitized sheet to form an image on the sensitized sheet. Thisoptical printer apparatus will now be described with reference to FIG.16.

A sensitized sheet 60 is driven at constant speed in the direction ofarrow Z with respect to the optical head 10 by means of feed rollers 70.The optical head 10 comprises a white light source 20 for radiallyemitting white light, a cylindrical lens 30 for linearly converging thewhite light on the sensitized sheet 60, a three-color separation liquidcrystal shutter 40, and a liquid crystal shutter 50.

The three-color separation liquid crystal shutter 40 is composed ofthree shutters 40 r, 40 g and 40 b that linearly extend in the widthdirection (spreading direction) of the white light from the cylindricallens 30. These three shutters 40 r, 40 g and 40 b are drivenindependently of one another, and are provided individually with colorfilters that transmit red (R), green (G), and blue (B) light beams,respectively.

The liquid crystal shutter 50 includes a plurality of pixels that arearranged in the same direction as the lengthwise direction of theshutters 40 r, 40 g and 40 b.

The following is a description of a method for forming an image on thesensitized sheet 60 by means of the apparatus shown in FIG. 16.

The optical printer apparatus receives gradated color image data,controls the shutters 40 r, 40 g and 40 b in accordance with the imagedata, and exposes the surface of the sensitized sheet 60, therebyforming the image thereon. After the shutter 40 r opened, the shutter 40g opens for a predetermined time, and after the shutter 40 g opened, theshutter 40 b opens for a predetermined time, to transmit the whitelight. This predetermined time is just equal to a period of time duringwhich the sensitized sheet 60 moves for a distance X in FIG. 16.

Thus, the sensitized sheet 60 is exposed to the red light beam (R),which is first transmitted through the shutter 40 r, for the distance Xin its moving direction (direction Z). Then, the shutter 40 r is closed,while the shutter 40 g opens. Since the sensitized sheet 60 is moved forthe distance X by this time, that portion of the sensitized sheet 60which has already been exposed to the light beam R is exposed again tothe green light beam (G) that is transmitted through the shutter 40 g.When the sensitized sheet 60 further moves for the distance X,thereafter, the portion already exposed to the light beams R and G isexposed in like manner to the blue light beam (B) that is transmittedthrough the shutter 40 b. By repeating these processes of operation inthe feeding direction of the sensitized sheet 60, an image of full-colordisplay can be obtained.

In a direction perpendicular to the feeding direction of the sensitizedsheet 60, an image is formed by means of the liquid crystal shutter 50.

Referring now to FIG. 17, there will be described exposure timing forthe formation of an image by means of the conventional optical printerapparatus shown in FIG. 16.

In FIG. 17, it is supposed, for ease of illustration, that thesensitized sheet 60 is stationary and the optical head 10 moves in thedirection of arrow Z. In order to indicate the color, R, G or B, of thelight beam to which the sensitized sheet 60 is exposed, moreover, thesensitized sheet 60 is divided into three layers for convenience.Exposure of the sensitized sheet 60 to the light beam R is representedby the hatching on the first layer from the top, among the aforesaidthree layers. Likewise, exposure to the light beam G and exposure to thelight beam B are represented by hatching the second and third layers,respectively. It is, to be understood that FIG. 17 never illustrates thefact that the actual sensitized sheet 60 is composed of those threelayers.

Sections {circle around (1)} to {circle around (6)} individuallyrepresent pixels in the moving direction (direction Z in FIG. 17) of theoptical head. The width of each pixel is represented by X in FIG. 17.

Item (a) of FIG. 17 shows a state in which the light beam R starts to beradiated so that the optical head 10 exposes the section {circle around(3)} on the sensitized sheet 60 thereto. As this is done, the lightbeams G and B are not radiated. Then, the optical head 10 radiates thelight beam R as it moves at uniform speed for the distance X (equal tothe pixel width) in the direction of arrow Z. The exposure of thesection {circle around (3)} to the light beam R terminates when theposition of (b) of FIG. 17 is reached.

The moment the optical head 10 comes to the position of (b) of FIG. 17to finish the radiation of the light beam R, the optical head 10 startsto radiate the light beam G for the section {circle around (3)}, asshown in (c) of FIG. 17. The section {circle around (3)} has alreadybeen exposed to the light beam R, as described above. Then, the opticalhead 10 radiates the light beam G as it moves at uniform speed for thedistance X in the direction of arrow Z. The exposure of the section{circle around (3)} to the light beam G terminates when the optical head10 comes to the position of (d) of FIG. 17.

The moment the optical head 10 comes to the position of (d) of FIG. 17to finish the radiation of the light beam G, the optical head 10 startsto radiate the light beam B for the section {circle around (3)}, asshown in (e) of FIG. 17. The section {circle around (3)} has alreadybeen exposed to the light beams R and G, as described above. Then, theoptical head 10 radiates the light beam B as it moves at uniform speedfor the distance X in the direction of arrow Z. The exposure of thesection {circle around (3)} to the light beam B terminates when theoptical head 10 comes to the position of (f) of FIG. 17.

As described above, the section {circle around (3)} of the sensitizedsheet 60 is exposed to the light beams R, G and B in a series ofprocesses of operation shown in (a) to (f) of FIG. 17. This series ofoperation processes will hereinafter be referred to as an exposurecycle. In a second exposure cycle subsequent to this cycle, the section{circle around (6)} is exposed, as shown in (g) of FIG. 17.

In the conventional optical printer, as described above, a full-colorimage can be formed on the sensitized sheet 60 by continuously repeatingthe aforesaid exposure cycles.

According to the conventional optical printer apparatus arranged in thismanner, however, the image pitch or spacing between images is equal to amaximum exposure distance (mentioned later), as mentioned before, sothat the position of the section {circle around (3)}, which is situatedat a distance 2X from the exposed section {circle around (3)}, isexposed between the first and second exposure cycles, as shown in (g) ofFIG. 6.

Thus, according to the conventional optical printer apparatus, the imageinvolves an unexposed portion (i.e., sections {circle around (4)} and{circle around (5)} that is twice as long as the exposure distance Xbetween the exposure cycles, resulting in lowered resolution and imagequality.

DISCLOSURE OF THE INVENTION

The object of the present invention is to provide an optical printerapparatus, capable of printing high-resolution, high-quality colorimages free from unexposed portions.

In order to achieve the above object, an optical printer apparatusaccording to the present invention comprises an optical head, capable ofradiating a plurality of color light beams while moving relatively to asensitized material, and a drive unit for driving the optical headand/or the sensitized material in order to cause the optical head andthe sensitized material to move relatively to each other at constantspeed, and is designed so that individual images formed on thesensitized material when the color light beams are radiatedsimultaneously are arranged at given pitches in the direction of therelative movement when the optical head is stationary with respect tothe sensitized material, and that an image is formed on the sensitizedmaterial as the light beams are applied in regular order accompanyingthe relative movement of the optical head. Let P be the image pitch ofthe color light beams on the sensitized material and D be the maximumexposure distance corresponding to the maximum emission time of thecolor light beams for each pixel, the maximum exposure distance D is setsmaller than the image pitch P.

According to the present invention, the whole area of the sensitizedmaterial can be exposed even in the case where the color light beams onthe sensitized material cannot be focused in close vicinity to oneanother in the moving direction of the optical head, so that theresolution of the image can be improved. Since the gradation of a regionbetween each two adjacent pixels is the average of the respectivegradations of the pixels, a fine image with good color mixture can beobtained.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a perspective view showing an outline of an optical printerapparatus according to the present invention;

FIG. 1B is a schematic view of the optical printer apparatus of FIG. 1A;

FIG. 2 is a diagram for illustrating the principle of gradation controlfor the optical printer apparatus according to the present invention;

FIG. 3 is a diagram for illustrating exposure timing for an 10 opticalprinter apparatus according to a first embodiment of the presentinvention to expose a sensitized sheet, showing first and secondexposure cycles;

FIG. 4 is the continuation of FIG. 3, showing third and fourth exposurecycles;

FIG. 5 is a diagram for illustrating exposure timing for an opticalprinter apparatus according to a second embodiment of the presentinvention to expose a sensitized sheet, showing first and secondexposure cycles;

FIG. 6 is the continuation of FIG. 5, showing third and fourth exposurecycles;

FIG. 7 is a diagram for illustrating exposure timing for an opticalprinter apparatus according to a third embodiment of the presentinvention to expose a sensitized sheet, showing first and secondexposure cycles;

FIG. 8 is the continuation of FIG. 7, showing third and fourth exposurecycles;

FIG. 9 is the continuation of FIG. 8, showing a fifth exposure cycle;

FIG. 10 is a diagram for illustrating exposure timing for an opticalprinter apparatus according to a fourth embodiment of the presentinvention to expose a sensitized sheet;

FIG. 11 is a diagram for illustrating exposure timing for an opticalprinter apparatus according to a fifth embodiment of the presentinvention to expose a sensitized sheet, showing first and secondexposure cycles;

FIG. 12 is the continuation of FIG. 11, showing third and fourthexposure cycles;

FIG. 13 is a diagram for illustrating exposure timing for an opticalprinter apparatus according to a sixth embodiment of the presentinvention to expose a sensitized sheet, showing first and secondexposure cycles;

FIG. 14 is the continuation of FIG. 13, showing third and fourthexposure cycles;

FIG. 15 is the continuation of FIG. 13, showing a fifth exposure cycle;

FIG. 16 is a schematic sectional view of a conventional optical printerapparatus; and

FIG. 17 is a diagram for illustrating exposure timing for theconventional optical printer apparatus to expose a sensitized sheet.

BEST MODE FOR CARRYING OUT THE INVENTION

First, the principal part of an optical printer apparatus will bedescribed with reference to FIGS. 1A and 1B.

An optical head 100 contains therein an optical system that is composedof a paraboloidal mirror 120, a cylindrical lens 130, and a reflector140 as well as an LED array 110. The optical head 100 is driven in thedirection of arrow z1 with respect to sensitized sheet 500 by means ofhead feeding means 300 (mentioned later).

The LED array 110 is composed of two rows of LED elements that emit red(R), green (G), and blue (B) light beams, each row including two LEDelements. The LED elements for R, G and B are vertically arranged in thedescending order on a photosensitive surface 500 a of the sensitizedsheet 500. Light beams emitted from the LED array 110 pass through thelower half of the cylindrical lens 130 and are reflected by theparaboloidal mirror 120, thus becoming parallel light beams. Theparallel light beams reflected by the paraboloidal mirror 120 passthrough the upper half of the cylindrical lens 130 and are reflected bythe reflector 140. They then advances at right angles to thephotosensitive surface 500 a of the sensitized sheet 500, pass through aliquid crystal shutter 150, and are focused on the photosensitivesurface 500 a. Thus, the focus of each light beam transmitted throughthe upper half of the cylindrical lens 130 is located on thephotosensitive surface 500 a of the sensitized sheet 500.

The liquid crystal shutter 150 includes one scanning electrode and 640signal electrodes, whereby 640 pixels are formed in a line in the widthdirection (direction indicated by arrow z2 in FIG. 1A) of the sensitizedsheet 500.

The head feeding means 300 includes an endless optical head scanningwire 373, pulleys 371 and 372 wound with the scanning wire 373, and a DCmotor 310 for rotating the pulley 371. A part of the scanning wire 373is fixed to a wire fixing portion 111 that protrudes from a side face ofthe optical head 100.

A fin 321 of a rotary encoder 320 is mounted on the rotating shaft ofthe DC motor 310. A large number of apertures 322 are formed in the fin321. A light emitting element and a light receiving element (not shown)of a photo-interrupter 323 face each other with the fin 321 betweenthem. The fin 321 and the photo-interrupter 323 constitute the rotaryencoder 320.

The fin 321 rotates simultaneously with the DC motor 310. As the fin 321rotates, the apertures 322 allow intermittent transfer of the lightbeams between the light emitting and receiving elements of thephoto-interrupter 323. An electrical signal is outputted in synchronismwith this intermittent transfer of the light beams, whereupon therotational angular position of the DC motor 310 is detected.

As shown in FIG. 1A, the rotational speed of the DC motor 310 is reducedby means of a worm gear 350 and gears 361, 362 and 363, and is convertedinto a linear reciprocation by means of the pulleys 371 and 372 and thescanning wire 373. The reciprocation of the scanning wire 373 causes thewire fixing portion 111 to move the optical head 100 in its scanningdirection.

A pair of position sensors 210 and 220, formed of a photo-interruptereach, are fixed to a substrate 230 of the optical printer apparatus.When a douser 240 that is fixed to the optical head 100 moves togetherwith the optical head 100 in the scanning direction, any one of or bothof the position sensors 210 and 220 are screened from light, whereuponthe position of the optical head 100 is detected.

In FIG. 1B, reference numeral 375 denotes a base plate of the opticalprinter apparatus. The base plate 375 contains therein the sensitizedsheet 500, a developing roller 376, a control circuit 377, etc.

The following is a description of a method for forming an image on thesensitized sheet 500.

The LED array 110 emits red, green, and blue light beams in thedescending order. The light beams from the LED array 110 spread in thetransverse direction (direction indicated by arrow z2 in FIG. 1A) asthey pass through the lower half of the cylindrical lens 130 and reachthe paraboloidal mirror 120. The light beams reflected by theparaboloidal mirror 120 and spread in the transverse direction areconverted into parallel light beams, and pass through the upper half ofthe cylindrical lens 130. The upper half of the cylindrical lens 130serves to converge the light beams reflected by the paraboloidal mirror120 and form an image with a given width on the plane of the sensitizedsheet 500.

The light beams converged by the upper half of the cylindrical lens 130are made to change their courses substantially at 90 degrees by the flatreflector 140, and start to advance at right angles to the plane of thesensitized sheet 500. Then, the light beams pass through the liquidcrystal shutter 150, and the sensitized sheet 500 is exposed to them.

The light beams focused with the given width on the sensitized sheet 500are arranged rearward in the order of R, G and B in the scanningdirection (direction z1), as shown in FIG. 1A.

When the optical head 100 is fed at a given speed in the scanningdirection (direction of arrow z1) by the head feeding means 300, thedouser 111 intercepts both light beams from the photo-interrupters 210and 220. Thereupon, it is concluded that the optical head 100 is in itswrite start position, and writing is started.

The following is a description of basic operation for writing.

First, the light beam R passes for a first time that is controlled bymeans of the liquid crystal shutter 150, whereby a predetermined regionof the sensitized sheet 500 is exposed. Then, the light beam G passesfor a second time that is controlled by means of the liquid crystalshutter 150, whereby that region is exposed. Further, the light beam Bpasses for a third time that is controlled by means of the liquidcrystal shutter 150, whereby the same region is exposed. Thus, afull-color image is formed on the aforesaid region.

These light beams of the three colors, R, G and B are expected to beapplied accurately to a predetermined position on the sensitized sheet500 in accordance with image data. Accordingly, the emission timing ofthe LED array 110 and the open-close timing of the liquid crystalshutter 150 are synchronized with the output of the rotary encoder 320that is mounted on the rotating shaft of DC motor 310.

Referring now to FIG. 2, there will be described gradation controlcarries out by the optical printer apparatus shown in FIGS. 1A and 1B.FIG. 2 shows the relation of the exposure time to the exposure distanceon the photosensitive surface 500 a of the sensitized sheet 500.

According to FIG. 2, the liquid crystal shutter 150 is closed when theoptical head is advanced for the distance D in the z-direction to forman image A2 with a width W on the sensitized sheet surface 500 a afterthe light beam R radiated from the liquid crystal shutter 150 forms animage A1 with the width W on the sensitized sheet surface 500 a.

Thereupon, the relation of the exposure time to the position indicatedby the exposure distance on the photosensitive surface 500 a of thesensitized sheet 500 is represented by a trapezoid B with a height oft1, as shown in FIG. 2. A section E of the photosensitive surface 500 acorresponding to the top side of the trapezoid B is a region thatcontinues to be exposed for a period of time t1 from the start ofexposure to the light beam R to the end of exposure. The exposure timet1 is a value obtained by dividing the distance D of movement by themoving speed (fixed value) of the optical head.

Thus, the exposure time is proportional to the distance D of movement. Amaximum exposure time or maximum gradation is obtained when the distanceD of movement has its maximum value. In the description to follow, thedistance D of movement for this maximum gradation will be referred to as“maximum exposure distance.”

In the regions of sections D and F that adjoin the section E, moreover,the exposure time linearly changes from 0 to t1 or from t1 to 0, so thatthe gradation on the sensitized sheet surface changes according to theexposure distance in the sections E and D.

An intermediate gradation is obtained in the case where the exposuredistance is not longer than the maximum exposure distance D. After thelight beam R radiated from the liquid crystal shutter 150 forms theimage A1 with the width W on the sensitized sheet surface 500 a, theoptical head is advanced for a distance d (<D). When an image A3 withthe width W is formed on the sensitized sheet surface 500 a, the liquidcrystal shutter 150 is closed. Thereupon, the relation of the exposuretime to the position indicated by the exposure distance on thephotosensitive surface 500 a of the sensitized sheet 500 is representedby a trapezoid C with a height of t2 (<t1), as shown in FIG. 2. Then, agradation corresponding to the exposure time t2 is given.

In the optical printer apparatus shown in FIGS. 1A and 1B, as describedabove, the exposure time t2 or gradation can be changed by changing theexposure distance d.

The following is a description of several examples of exposure timingfor the exposure of the sensitized sheet 500 by means of the opticalprinter apparatus.

First Embodiment: FIGS. 3 and 4

A first embodiment will be described with reference to FIGS. 3 and 4. InFIGS. 3 and 4, the optical head moves at uniform speed in the directionof arrow Z with respect to the sensitized sheet 500. Then, the lightbeams R, G and B radiated from the optical head are indicated by twofull-line arrows that are directed toward the sensitized sheet 500.Dotted-line arrows indicate the respective positions of the light beamsafter movement for the maximum exposure distance.

The hatching between the two full-line arrows for R, G or B indicatethat the light beam R, G or B is in a radiation start position. On theother hand, the hatching between the two dotted-line arrows for R, G orB indicates that the light beam R, G or B is in a radiation end positionwhere it moved by the maximum exposure distance from the radiation startposition. Thus, the region in which the hatching between the twofull-line arrows for R, G or B and the hatching between the twodotted-line arrows are superposed corresponds to the region E shown inFIG. 2, in which the exposure time is t1 and the maximum gradation isgiven.

In order to indicate the color, R, G or B, of the light beam to whichthe sensitized sheet 500 is exposed, moreover, the sensitized sheet 500is divided into three layers for convenience, as described in connectionwith the prior art example shown in FIG. 17. Exposure to the light beamR is represented by the hatching on the first layer from the top,exposure to the light beam G by the hatching on the second layer, andexposure to the light beam B by the hatching on the third layer from thetop.

Sections {circle around (1)} to {circle around (8)} individuallyrepresent pixels in the scanning direction of the optical head.

As shown in (a) of FIG. 3, the light beams R, G and B individually formimages with the width W on the sensitized sheet 500. These images arearranged at equal spaces in the scanning direction (direction Z shown in(a) of FIG. 3) of the optical head. The layout pitch (image pitch) forthe images is indicated by P in (a) of FIG. 3. The image width W istwice as long as the maximum exposure distance D.

The size of the image pitch P is settled by P=(NC+1)D. In thisexpression, C is the number of color light beams. In the case of thepresent embodiment, C=3, as three colors R, G and B are used. D is themaximum exposure distance. N is a positive integer (N=1, 2 . . . ). Inthe present embodiment, N=1 is selected, so that P=4D is obtained.

(First Exposure Cycle: (a) to (c) of FIG. 3)

(a) Exposure of Section {circle around (4)} to Light Beam R: The lightbeam R starts to be radiated in the position indicated by full-linearrows, and the section {circle around (4)} is then exposed thereto asthe light beam R moves to the position indicated by dotted-line arrows,that is, for the maximum exposure distance D, whereupon the radiationterminates. As this is done, the light beams G and B are not radiated.With this radiation of the light beam R, only a sensitizing agent thatis applied to the sensitized sheet 500 and reacts to the light beam R isexposed. This exposure is represented by hatching the first layer fromthe top of the section {circle around (4)} of the sensitized sheet 500.

(b) Exposure of Section {circle around (3)} to Light Beam G: The momentthe radiation of the light beam R is finished, the light beam G startsto be radiated in the position indicated by full-line arrows, and thesection {circle around (3)} is then exposed thereto as the light beam Gmoves to the position indicated by dotted-line arrows, that is, for themaximum exposure distance D, whereupon the radiation terminates. Thisexposure is represented by hatching the second layer from the top of thesection {circle around (3)} of the sensitized sheet 500.

(c) Exposure of Section {circle around (2)} to Light Beam B: The momentthe radiation of the light beam G is finished, the light beam B startsto be radiated in the position indicated by full-line arrows, and thesection {circle around (2)} is then exposed thereto as the light beam Bmoves to the position indicated by dotted-line arrows, that is, for themaximum exposure distance D, whereupon the radiation terminates. Thisexposure is represented by hatching the third layer from the top of thesection {circle around (2)} of the sensitized sheet 500.

Thus, each cycle of emission of R, G and B shown in (a) to (c) of FIG. 3constitutes one exposure cycle. This exposure cycle is repeated manytimes to expose the sensitized sheet 500, whereupon an image is formedon the surface of the sensitized sheet.

In the one exposure cycle, as described above, each light beam continuesto be emitted (that is, the maximum exposure time is given, and themaximum gradation is given to each section) while it moves for themaximum exposure distance D. Actually, however, the gradation of eachlight beam is controlled, so that the maximum exposure time is notalways given. In the case where the gradation is controlled, asmentioned before, the radiation distance (radiation time) is adjusted byclosing the liquid crystal shutter 150 halfway with the light notradiated throughout the maximum exposure distance D. Thus, the exposuredistance (exposure time) is adjusted.

As described above, different sections on the sensitized sheet 500 areexposed to the light beams, individually, in each exposure cycle. Morespecifically, the sections {circle around (4 )}, {circle around (3)} and{circle around (3)} are exposed to the light beams R, G and B,respectively, in a first exposure cycle. Thus, the image data aredesigned to control the radiation distance in the section {circle around(3)}, the radiation distance in the (adjacent) section {circle around(3)}, and the radiation distance in the (adjacent) section {circlearound (2)}, individually.

(Second Exposure Cycle: (d) to (f) of FIG. 3)

(d) Exposure of Section {circle around (5)} to Light Beam R: The momentthe radiation of the light beam B (see (c) of FIG. 3) is finished, thelight beam R starts to be radiated in the position indicated byfull-line arrows, and the section {circle around (5)} is then exposedthereto as the light beam R moves to the position indicated bydotted-line arrows, that is, for the maximum exposure distance D,whereupon the radiation terminates.

(e) Exposure of Section {circle around (4)} to Light Beam G: The momentthe radiation of the light beam R is finished, the light beam G startsto be radiated in the position indicated by full-line arrows, and thesection {circle around (4)} is then exposed thereto as the light beam Gmoves to the position indicated by dotted-line arrows, that is, for themaximum exposure distance D, whereupon the radiation terminates.

(f) Exposure of Section {circle around (3)} to Light Beam B: The momentthe radiation of the light beam G is finished, the light beam B startsto be radiated in the position indicated by full-line arrows, and thesection {circle around (3)} is then exposed thereto as the light beam Bmoves to the position indicated by dotted-line arrows, that is, for themaximum exposure distance D, whereupon the radiation terminates.

(Third Exposure Cycle: (g) to (i) of FIG. 4)

(g) Exposure of Section {circle around (6)} to Light Beam R: Explanationis omitted here and in the following.

(h) Exposure of Section {circle around (5)} to Light Beam G:

(i) Exposure of Section {circle around (4)} to Light Beam B: When theexposure of section {circle around (4)} to the light beam B is finished,the section {circle around (4)} can be concluded to have been exposed toall the light beams R, G and B.

(Fourth Exposure Cycle: (j) to (l) of FIG. 4)

(j) Exposure of Section {circle around (7)} to Light Beam R:

(k) Exposure of Section {circle around (6)} to Light Beam G:

(l) Exposure of Section {circle around (5)} to Light Beam B: When theexposure of section {circle around (5)} to the light beam B is finished,the section {circle around (5)} can be concluded to have been exposed toall the light beams R, G and B.

In a fifth exposure cycle (not shown), moreover, the section {circlearound (6)} is exposed to all the light beams R, G and B. In thismanner, the sections exposed to all the light beams R, G and B increaseone by one in the scanning direction (direction Z) of the optical headwith every exposure cycle. Thus, the whole surface of the sensitizedsheet is exposed to the light beams of the three primary colors havinggradations, whereby a full-color image is formed.

Second Embodiment: FIGS. 5 and 6

A second embodiment will be described with reference to FIGS. 5 and 6.This embodiment differs from the first embodiment shown in FIGS. 3 and 4only in that the width W of the image of the color light beams R, G andB on the sensitized sheet 500 is three times (twice in the firstembodiment) as long as the maximum exposure distance D. The image pitchP and the maximum exposure distance D has the same relation, P=4D, as inthe first embodiment.

Since the image width W according to the second embodiment is greaterthan that according to the first embodiment, however, the exposuresections overlap one another for a margin corresponding to the maximumexposure distance D in the manner described below. (The exposuresections never overlap one another in the first embodiment.)

(First Exposure Cycle: (a) to (c) of FIG. 5)

(a) Exposure of Section {circle around (4)} to Light Beam R: The imagewidth W is so great that ⅓ of the adjacent section {circle around (3)},as well as the section {circle around (4)}, is exposed to the light beamR.

(b) Exposure of Section {circle around (3)} to Light Beam G: Likewise, ⅓of the adjacent section {circle around (2)}, as well as the section{circle around (3)}, is exposed to the light beam G. A third of thesection {circle around (3)} that is nearer to the section {circle around(4)} is exposed to both the light beams G and R.

The width of the region that is doubly exposed to those two light beamscan be adjusted by changing the image width W By changing thecorrespondence between the image data and the actual image, moreover,the section {circle around (3)} and ⅓ of its adjacent section {circlearound (4)} (on the side remoter from {circle around (2)} can be made tobe exposed to the light beam G. A high-quality image with improved colormixture between pixels can be formed by superposing the exposuresections in this manner.

(c) Exposure of Section {circle around (2)} to Light Beam B: Likewise, ⅓of the adjacent section {circle around (1)}, as well as the section{circle around (2)}, is exposed to the light beam B. A third of thesection {circle around (2)} that is nearer to the section {circle around(3)} is exposed to the two light beams B and G.

(Second Exposure Cycle: (d) to (f) of FIG. 5)

(d) Exposure of Section {circle around (5)} to Light Beam R: Explanationis omitted here and in the following.

(e) Exposure of Section {circle around (4)} to Light Beam G:

(f) Exposure of Section {circle around (3)} to Light Beam B:

(Third Exposure Cycle: (g) to (i) of FIG. 6)

(g) Exposure of Section {circle around (6)} to Light Beam R:

(h) Exposure of Section {circle around (5)} to Light Beam G:

(i) Exposure of Section {circle around (4)} to Light Beam B: When thisexposure is finished, the section {circle around (4)} can be concludedto have been exposed to all the light beams R, G and B.

(Fourth Exposure Cycle: (j) to (l) of FIG. 6)

(j) Exposure of Section {circle around (7)} to Light Beam R:

(k) Exposure of Section {circle around (6)} to Light Beam G:

(l) Exposure of Section {circle around (5)} to Light Beam B: When thisexposure is finished, the section {circle around (5)} can be concludedto have been exposed to all the light beams R, G and B.

By further repeating the exposure cycles described above, the section{circle around (6)} and the subsequent sections are successively exposedto the light beams R, G and B. Thus, a full-color image can be obtainedwithout involving unexposed regions between pixels on the sensitizedsheet.

Third Embodiment: FIGS. 7, 8 and 9

A third embodiment will be described with reference to FIGS. 7, 8 and 9.The third embodiment differs from the foregoing second embodiment (C=3,N=1; P=4D) in that the aforementioned expression P=(NC+1)D is rewrittenas P=7D based on C=3 and N=2.

(First Exposure Cycle: (a) to (c) of FIG. 7)

(a) Exposure of Section {circle around (6)} to Light Beam R: Explanationis omitted here and in the following.

(b) Exposure of Section {circle around (4)} to Light Beam G:

(c) Exposure of Section {circle around (2)} to Light Beam B:

(Second Exposure Cycle: (d) to (f) of FIG. 7)

(d) Exposure of Section {circle around (7)} to Light Beam R:

(e) Exposure of Section {circle around (5)} to Light Beam G:

(f) Exposure of Section {circle around (3)} to Light Beam B:

(Third Exposure Cycle: (g) to (i) of FIG. 8)

(g) Exposure of Section {circle around (8)} to Light Beam R:

(h) Exposure of Section {circle around (6)} to Light Beam G:

(i) Exposure of Section {circle around (4)} to Light Beam B:

(Fourth Exposure Cycle: (j) to (l) of FIG. 8)

(j) Exposure of Section {circle around (9)} to Light Beam R:

(k) Exposure of Section {circle around (7)} to Light Beam G:

(l) Exposure of Section {circle around (5)} to Light Beam B:

(Fifth Exposure Cycle: (m) to (o) of FIG. 9)

(m) Exposure of Section {circle around (10)} to Light Beam R:

(n) Exposure of Section {circle around (8)} to Light Beam G:

(o) Exposure of Section {circle around (6)} to Light Beam B: When thisexposure is finished, the section {circle around (6)} can be concludedto have been exposed to all the light beams R, G and B.

By further repeating the exposure cycles described above, the section{circle around (7)} and the subsequent sections are successively exposedto the light beams R, G and B. Thus, a full-color image can be obtainedwithout involving unexposed regions between pixels on the sensitizedsheet.

Fourth Embodiment: FIG. 10

A fourth embodiment will be described with reference to FIG. 10. In thefourth embodiment, the number of colors is not 3 but two (R and G).Accordingly, the aforementioned expression P=(NC+1)D is rewritten asP=5D based on C=2 and N=2.

(a) Exposure of Section {circle around (3)} to Light Beam R: Explanationis omitted here and in the following.

(b) Exposure of Section {circle around (2)} to Light Beam G:

(c) Exposure of Section {circle around (4)} to Light Beam R:

(d) Exposure of Section {circle around (2)} to Light Beam G:

(e) Exposure of Section {circle around (4)} to Light Beam R:

(f) Exposure of Section {circle around (3)} to Light Beam G:

Fifth Embodiment: FIGS. 11 and 12

A fifth embodiment will be described with reference to FIGS. 11 and 12.In the fifth embodiment, the number of colors is not 3 (R, G and B) butfour (R, G, B1 and B2). In this case, B is divided into B1 and B2 sothat a predetermined exposure intensity for B on the sensitized sheetcan be obtained by exposing the sheet twice with the light beams B1 andB2, since the light beam B is feeble due to the characteristics of theLED. Accordingly, the image pitch, which is given by the aforementionedexpression P=(NC+1)D, is rewritten as P=5D based on C=4 and N=1.

(First Exposure Cycle: (a) to (d) of FIG. 11)

(d) Exposure of Section {circle around (4)} to Light Beam R: Explanationis omitted here and in the following.

(e) Exposure of Section {circle around (3)} to Light Beam G:

(f) Exposure of Section {circle around (2)} to Light Beam B1:

(g) Exposure of Section {circle around (1)} to Light Beam B2:

(Second Exposure Cycle: (e) to (h) of FIG. 11)

(e) Exposure of Section {circle around (5)} to Light Beam R:

(f) Exposure of Section {circle around (4)} to Light Beam G:

(g) Exposure of Section {circle around (3)} to Light Beam B1:

(h) Exposure of Section {circle around (2)} to Light Beam B2:

(Third Exposure Cycle: (i) to (l) of FIG. 12)

(i) Exposure of Section {circle around (6)} to Light Beam R:

(j) Exposure of Section {circle around (5)} to Light Beam G:

(k) Exposure of Section {circle around (4)} to Light Beam B1:

(l) Exposure of Section {circle around (3)} to Light Beam B2:

(Fourth Exposure Cycle: (m) to (p) of FIG. 12)

(m) Exposure of Section {circle around (7)} to Light Beam R:

(n) Exposure of Section {circle around (6)} to Light Beam G:

(o) Exposure of Section {circle around (5)} to Light Beam B1:

(p) Exposure of Section {circle around (4)} to Light Beam B2: When thisexposure is finished, the section {circle around (4)} is exposed to thefour color light beams R, G, B1 and B2.

By further repeating the exposure cycles described above, the section{circle around (5)} and the subsequent sections are successively exposedto the four color light beams. Thus, a full-color image can be obtainedwithout involving unexposed regions between pixels on the sensitizedsheet.

Sixth Embodiment: FIGS. 13, 14 and 15

A sixth embodiment will be described with reference to FIGS. 13, 14 and15. The sixth embodiment differs from the foregoing embodiments in thatthe light beams from the LED are emitted in the direction opposite tothe moving direction Z of the optical head. The three color light beamsR, G and B are arranged in the moving direction Z of the optical head inthe order named.

In this case, the relation between the image pitch P and the maximumexposure distance D is given by P=(NC−1)D, which is rewritten as P=5Dbased on N=2 and C=3 (R, G and B).

(First Exposure Cycle: (a) to (c) of FIG. 13)

(a) Exposure of Section {circle around (1)} to Light Beam B: Explanationis omitted here and in the following.

(b) Exposure of Section {circle around (3)} to Light Beam G:

(c) Exposure of Section {circle around (5)} to Light Beam R:

(Second Exposure Cycle: (d) to (f) of FIG. 13)

(d) Exposure of Section {circle around (2)} to Light Beam B:

(e) Exposure of Section {circle around (4)} to Light Beam G:

(f) Exposure of Section {circle around (6)} to Light Beam R:

(Third Exposure Cycle: (g) to (i) of FIG. 14)

(g) Exposure of Section {circle around (3)} to Light Beam B:

(h) Exposure of Section {circle around (5)} to Light Beam G:

(i) Exposure of Section {circle around (7)} to Light Beam R:

(Fourth Exposure Cycle: (j) to (l) of FIG. 14)

(j) Exposure of Section {circle around (4)} to Light Beam B:

(k) Exposure of Section {circle around (6)} to Light Beam G:

(l) Exposure of Section {circle around (8)} to Light Beam R:

(Fifth Exposure Cycle: (m) to (o) of FIG. 15)

(m) Exposure of Section {circle around (5)} to Light Beam B: When thisexposure is finished, the section {circle around (5)} is exposed to thelight beams R, G and B.

(n) Exposure of Section {circle around (7)} to Light Beam G:

(o) Exposure of Section {circle around (9)} to Light Beam B:

By further repeating the exposure cycles described above, the section{circle around (6)} and the subsequent sections are successively exposedto the light beams R, G and B. Thus, a full-color image can be obtainedwithout involving unexposed regions between pixels on the sensitizedsheet.

What is claimed is:
 1. An optical printer apparatus which comprises anoptical head capable of radiating a plurality of color light beams whilemoving relatively to a sensitized material, and a drive unit for drivingat least one of the optical head and the sensitized material in order tocause the optical head and the sensitized material to move relatively toeach other at constant speed, wherein the optical printer apparatus isdesigned so that individual images formed on the sensitized materialwhen said plurality of color light beams are radiated simultaneously arearranged at given pitches in the direction of said relative movementwhen the optical head is stationary with respect to the sensitizedmaterial, and wherein an image is formed on the sensitized material asthe light beams are applied in a predetermined order during saidrelative movement of the optical head, characterized in that a maximumexposure distance D is smaller than an image pitch P, where the imagepitch P is the image pitch of said plurality of color light beams on thesensitized material and the maximum exposure distance D is the relativemovement distance of said optical head with respect to said sensitizedmaterial, which corresponds to the maximum exposure time of said colorlight beams.
 2. An optical printer apparatus according to claim 1,wherein said image pitch P is substantially equal to an integer multipleof said maximum exposure distance D.
 3. An optical printer apparatusaccording to claim 2, wherein said plurality of color light beams areapplied in a predetermined order in the direction of the movement ofsaid optical head relative to said sensitized material.
 4. An opticalprinter apparatus according to claim 3, wherein the relation between theimage pitch P and the maximum exposure distance D is given by P=(NC+1)D,where C is the number of color light beams and N is a positive integernot smaller than
 1. 5. An optical printer apparatus according to claim4, wherein said number C of color light beams is
 3. 6. An opticalprinter apparatus according to claim 5, wherein said color light beamsof three different colors include a red light beam, a green light beam,and a blue light beam.
 7. An optical printer apparatus according toclaim 6, wherein said positive integer N is 1, and the relation betweenthe image pitch P and the maximum exposure distance D is given by P=4D.8. An optical printer apparatus according to claim 4, wherein saidnumber C of color light beams is
 4. 9. An optical printer apparatusaccording to claim 8, wherein said color light beams of three differentcolors include a red light beam, a green light beam, and a blue lightbeam.
 10. An optical printer apparatus according to claim 2, whereinsaid plurality of color light beams are applied in an order in thedirection opposite to the direction of the movement of said optical headrelative to said sensitized material.
 11. An optical printer apparatusaccording to claim 10, wherein the relation between the image pitch Pand the maximum exposure distance D is given by P=(NC−1)D, where C isthe number of color light beams and N is a positive integer not smallerthan
 1. 12. An optical printer apparatus according to claim 11, whereinsaid number C of color light beams is
 3. 13. An optical printerapparatus according to claim 12, wherein said color light beams of threedifferent colors include a red light beam, a green light beam, and ablue light beam.
 14. An optical printer apparatus according to claim 13,wherein said arbitrary position integer N is 1, and the relation betweenthe image pitch P and the maximum exposure distance D is given by P=4D.15. An optical printer apparatus according to claim 14, wherein saidnumber C of color light beams is
 4. 16. An optical printer apparatusaccording to claim 5, wherein said color light beams of four differentcolors include a red light beam, a green light beam, and two blue lightbeams.
 17. An optical printer apparatus according to any one of claims 1to 16, wherein a light source for radiating said color light beams isformed of an LED (light emitting diode).
 18. An optical printerapparatus according to any one of claims 4 to 16, wherein said opticalhead carries out gradation control by controlling the exposure time ofeach pixel in accordance with gradated image data composed of aplurality of pixels, thereby forming a gradated image on said sensitizedmaterial.
 19. An optical printer apparatus according to claim 18,wherein, when any one of said plurality of color light beams undergoesgradation control in accordance with M-th pixel data, as counted in thedirection opposite to the moving direction of said optical head withrespect to said sensitized material, where M is a positive integer notsmaller than 1, and in the case where the color light beam is adjoinedby another color light beam in the moving direction of said opticalhead, said another color light beam undergoes gradation control inaccordance with (M+N)-th pixel data based on said positive integer N, ascounted in the direction opposite to the moving direction of saidoptical head, and in the case where the color light beam is adjoined byanother color light beam in the direction opposite to the movingdirection of said optical head, said another color light beam undergoesgradation control in accordance with (M−N)-th pixel data based on saidpositive integer N, as counted in the direction opposite to the movingdirection of said optical head.
 20. A color printing method in which thesurface of a sensitized sheet is divided into a plurality of regions (1,2, 3 . . . N . . . ) in the scanning direction of an optical head andgradations for first, second, and third colors are assigned for eachregion by image data, comprising steps of: (a) opening a shutter of theoptical head, radiating a light beam of a first color with a given widthtoward an N-th region on the sensitized sheet, then moving the opticalhead to move the light beam in the scanning direction of the opticalhead, and closing the shutter in a position reached by the light beamadvanced from a radiation start position by a distance d11 (≦D) assignedby the image data; (b) opening said shutter again after moving theoptical head by a distance which allows the light beam of the firstcolor to move for a preset distance D, radiating a light beam of asecond color with the width W toward an (N−1)-th region on thesensitized sheet, then moving the optical head to move the light beam inthe scanning direction of the optical head, and closing the shutter in aposition reached by the light beam advanced from the radiation startposition by a distance d12 (≦D) assigned by the image data; (c) openingsaid shutter again after moving the optical head by a distance whichallows the light beam of the second color to move by the preset distanceD, radiating a light beam of a third color with the width W toward an(N−2)-th region on the sensitized sheet, then moving the optical head tomove the light beam in the scanning direction of the optical head, andclosing the shutter in a position reached by the light beam advancedfrom the radiation start position by a distance d13 (≦D) assigned by theimage data; (d) opening said shutter again after moving the optical headby a distance which allows the third color light to move by the presetdistance D, radiating the light beam of the third color with the width Wtoward an (N+1)-th region on the sensitized sheet, then moving theoptical head to move the light beam in the scanning direction of theoptical head, and closing the shutter in a position reached by the lightbeam advanced from the radiation start position by a distance d21 (≦D)assigned by the image data; and (e) executing the same operationthereafter.
 21. An optical printer apparatus comprising: an optical headradiating a plurality of color light beams while moving relatively to asensitized material, and a drive unit for driving at least one of theoptical head and the sensitized material in order to cause the opticalhead and the sensitized material to move relatively to each other atconstant speed, wherein said color light beams are radiated one by oneon a time-sharing basis without simultaneous radiation among them, andin a predetermined order periodically in association with the relativemovement of said optical head with respect to said sensitized material,and an image width W of said color light beams on the sensitizedmaterial in the direction of said relative movement is smaller than amaximum exposure distance D which is the relative movement distance ofsaid optical head with respect to said sensitized material, and whichcorresponds to a maximum exposure time of said color light beams. 22.The optical printer apparatus according to claim 20, wherein said colorlight beams are of red (R), green (G), and blue (B) colors.